1. Field of the Invention
The present invention concerns a method to optimize a flow coding that is achieved by switching of an additional bipolar dephasing gradient pair that is used in a magnetic resonance phase contrast angiography measurement (data acquisition), wherein the strength of the flow coding is selected depending on the flow rate in the vessels with which the magnetic resonance phase contrast angiography measurement should be depicted. The invention furthermore concerns a MR system to implement the method.
2. Description of the Prior Art
In phase contrast angiography by means of nuclear magnetic resonance, the phase effects of flowing spins are used in order to depict them. These phase effects are based on the fact that, given a bipolar gradient pulse, excited spins that move along a magnetic field gradient have a different phase curve than resting spins. With resting spins, given switching of a bipolar gradient moment the phase is ideally zero after switching this bipolar gradient moment. This does not apply to moving spins since the spin is located in a different position than at the point in time of the negative gradient pulse than at the point in time of the associated positive gradient pulse. Overall, a phase shift that is directly proportional to the speed of the spins results after switching a bipolar gradient pulse. Such bipolar dephasing gradients are switched in MR phase contrast angiography, while the bipolar dephasing gradients are omitted in a further measurement. A difference signal that directly depends on the speed of the moving spins results via a complex difference calculation of the two magnetization vectors. The phase difference that results in such a manner between the overview measurement with bipolar dephasing gradient pair and without dephasing gradient pair is proportional to the speed and can thus be used for quantitative determination of flow velocities. The higher the flow velocity, the greater the phase shift. The greatest phase shift is achieved when the two magnetization vectors from the two overview measurements with and without bipolar dephasing gradient pair are opposite, meaning that the phase difference amounts to 180°. In addition to the flow velocity, the phase difference also depends on the gradient moment of the bipolar dephasing gradient pair, i.e. on the strength and the length of the dephasing gradient pair. Since only values between −180° and +180° can be shown as phase values, in the phase difference images phase jumps [discontinuities] can result if the selected gradient moment is too large. In order to determine the optimal strength of these bipolar dephasing gradients, overview measurements are applied with varying strengths of the bipolar dephasing gradients. Then, with consideration of the resulting phase difference images, a check is made as to when phase jumps occur in the vessels.
Since the flow velocity is typically also coded and compensated only in one spatial direction, the slice plane must be chosen with an adaptation to the vessel anatomy so that the flow takes place in the direction in which the bipolar dephasing gradient pair is switched to code the blood flow. For the user, this means a very time-consuming and laborious preparation, since what strength of the bipolar dephasing gradient pair is the correct one must be discovered according by trial-and-error.